In recent years, organic light-emitting diodes (OLEDs) have become increasingly important. This is due, at least in part, to the technological potential for OLED use in numerous products including, for example, multi-color, self-luminous flat-panel displays. An OLED exhibits several advantages over other light emitting devices. Some of these advantages include its wide range color emission capability, relatively low-voltage (e.g., <3V) operational capability, high-efficiency with low-power consumption, wide-viewing angle, and high-contrast.
A typical OLED includes an anode, a cathode, and at least two organic material layers disposed between the anode and cathode. The anode in many OLEDs comprises a relatively high work function material, such as indium tin oxide (ITO), and the cathode typically comprises a relatively low work function material, such as calcium (Ca). One of the organic material layers in a typical OLED comprises a material having the ability to transport holes, and is thus typically referred to as a hole transport layer. Another organic material layer typically comprises a material having the ability to transport electrons, and is thus typically referred to as an electron transport layer. The electron transport layer may also function as the luminescent medium (or emissive layer). Alternatively, an additional emissive layer may be disposed between the hole transport layer and the electron transport layer. In either case, when the OLED is properly biased, the anode injects holes (positive charge carriers) into the hole transport layer, and the cathode injects electrons into the electron transport layer. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, a Frenkel exciton is formed, and visible light is emitted.
While OLEDs have already appeared in some commercial applications, such as displays for mobile phones and digital cameras, certain challenges still remain to resolve various issues that adversely impact device reliability, chromaticity, and luminous efficiencies. For example, the surface roughness of anodes that comprise certain materials, such as indium tin oxide (ITO), contributes to the formation of dark spots, degradation, and eventual failure of many current OLEDs. As such, a significant amount of effort has been expended to improve OLED performance by modifying the anode surface. Some examples of anode surface modification techniques that have been attempted include chemical treatment, UV ozone treatment, oxygen plasma treatment, and mechanical polishing and annealing. In addition to these surface treatments, various other approaches have been attempted to address the adverse impact associated with anode surface roughness. These other approaches include depositing a layer of material, such as CuPc, LiF, Platinum, SiO2, metal oxide, or parylene, onto anode surface. Although such treatments and modifications may enhance hole injection from the ITO anode and improve the luminous efficiency of the OLED, these treatments and modifications do not sufficiently improve the life time of the OLED.
Hence, there is a need for an OLED that exhibits adequate performance, such as high luminous efficiency and long life time, so that OLEDs can be used in relatively high-demand applications. The present invention addresses at least this need.